When pipes for pipelines, gas-pipelines and similar grids formed by conduits have to cross geographical zones having irregular orography, or comprising rivers, inhabited areas, railway lines, motorways or other surface infrastructures, the conduits themselves have to pass beneath the ground.
To this end, special prefabricated tunnels are predisposed (in this context going by the name of microtunnels, abbreviated to MT), frequently defined by reinforced-concrete walls, each suitable for internally receiving a pipeline, and having an extension which is sufficient for complete crossing of the above-mentioned geographical zones by the pipeline.
In practice, the tunnel is provided with a mouth and an exit located at ground level, respectively for insertion and exit of the conduit previously or already realized in its entirety, or realized in a multiplicity of prefabricated sections which are then joined to form the whole pipeline.
At present, for laying or threading a pipe in the tunnel, a process as follows is used.
A head member provided with a hook is fixed to a first end of the pipe.
A cable is fastened to the hook, which cable must be of such a length as to be insertable in the mouth of the tunnel such as to pass through its total length up to emerging from the exit, then to be connected to a hoist.
The hoist is activated such that the cable draws the pipe into the tunnel up until it is completely threaded through it.
Experience has taught that this process is subject to a serious drawback, as follows.
The tunnel is usually constructed such as to describe a pathway that is substantially parabolic along the longitudinal development thereof, with an ideal vertex located below ground level.
In some not infrequent cases the tunnel also exhibits a drop in level between the entry and the exit.
In other words, a downstream part thereof can be arranged in descent with respect to an upstream part, and the tunnel can be said to be in descent.
In practice, the substantially parabolic pathway described above has an ideal vertex which is not the lowest point of the tunnel, but which is instead at a section of the tunnel downstream of the vertex, and at times in proximity of the exit.
In this case, during threading, at least a component of the velocity of the conduit is due to force of gravity, also in the sections of the tunnel that are close to the exit or which are in any case located downstream of the ideal vertex of the substantially parabolic pathway.
As the pipe is usually made of steel, and has significant dimensions, it is very heavy.
Owing to this, experience has shown that the in some cases the pipe can proceed internally of the tunnel even when the hoist is not performing traction, due to inertia and/or force of gravity.
This occurs not only in the sections close to the mouth, but also those downstream of the ideal vertex of the substantially parabolic pathway.
In these cases, its insertion in the tunnel is not controlled by the operators and the velocity the pipe proceeds at internally of the tunnel can consequently be unregulated by the operators.
The consequences of this eventuality can be very negative.
Indeed, not only is there the possibility that the conduit gets damaged during its uncontrolled sliding in the tunnel at an excessive velocity, there is especially the risk that the conduit will emerge from the tunnel at considerable speed, with the possible risk of damaging structures or machinery in the worksite that are predisposed downstream of the tunnel; not to mention the possible risk of injury to personnel.
TA further drawback of the known process is constituted by the fact that during use it is contemporaneously necessary both to perform the traction operation using the hoist located downstream of the tunnel close to the exit, and to control the insertion of the conduit, and possibly even to supply the various prefabricated sections if the conduit is not inserted as a whole part, upstream of the tunnel, in the environs of the mouth.
This implicates having to set up two different worksites, one upstream and one downstream of the tunnel, with a consequent waste of means and personnel, and with the difficulties required for coordinating operations which have to be synchronized but which are performed in different geographic localities.